1. Field of the Invention
The present invention relates to a head-up display device for the display of data aboard automobiles.
In particular the present invention relates to a HUD (Head-Up Display), which uses the windshield of an automobile as an optical element on which images are combined and replayed to the observer.
2. Description of the Prior Art
Head-up display devices (HUD) are commonly used in the field of military aeronautics in fighter planes, which permit the pilot to observe the in-flight scenario and at the same time to observe instruments, signals, etc., without having to move his eyes from the scene, thus reducing the risks connected with momentary lack of attention to the scenario visible from the front windscreen of the aircraft.
In the field of automobiles, it is equally useful to reduce the causes of distraction for the driver from observation of traffic and the road, due, for example, to keeping an eye on the speedometer, the rev counter, the direction indicator lights (indicators) and the like which are to be found aboard any automobile.
Head-up display devices of an aeronautical type are impractical for automobiles for a number of reasons, among which are included cost and size.
U.S. Pat. No. 4,740,780 to Braun et al. discloses a HUD-type device designed for use aboard automobiles. A similar arrangement is also illustrated and described in the published European patent application No. 0 229 876 in the name of SAZAKI Corporation.
The devices disclosed in these documents are complex and bulky and, as a consequence of the described conditions of use aboard automobiles, require among other things the use of aspherical optical components which are costly and difficult to produce.
In effect, as stated in the two documents mentioned above (the specifications of which are incorporated herein by reference in order to explain the problems involved more clearly), automobiles present problems with respect to the use of HUD devices which greatly differ from those presented by military aeronautics. Specifically, the optical element combining the HUD image and the surrounding scene is the windshield in automobiles, and it must be remembered that the boundary conditions (stereoscopic binocular vision, accommodation of the eye, and "projection" of the HUD image) are different in aeronautics and in commercial automobiles.
In an automobile, the reflection of the sun's light gives rise to problems of contrast which do not occur in military-type aeronautical applications.